The present invention relates to timebase generators, and more particularly to a phase-locked timebase for electro-optic sampling that synchronizes the sampling of the response of a device under test with a stimulus, and controls the delay between them.
In many test and measurement devices only the response of a device under test is available. In order to measure the response where it is a repetitive signal, a trigger signal, either external or internal, is generated to sample the response signal. The generation of the trigger signal has a certain minimum delay and a certain amount of delay jitter which causes sampling for a given point of the response signal to be offset from sample to sample, resulting in smearing of the value of the point and in the inability to sample the response before or near the trigger event. Averaging of the samples may be used to reduce the noise associated with this jitter. However, where the response is an impulse that is shorter in duration than the jitter of the trigger signal, the resulting averaged sample value is erroneous. Therefore it is advantageous to control both the stimulus as well as the sampling of the response for a device under test so that the requirement for generating the trigger signal may be eliminated.
Various stimulus/response systems have been developed to test a device under test. For example U.S. Pat. No. 4,434,399 issued to Mourou et al discloses an electrooptic sampling system that generates an optical pulse which is split (a) to energize a stimulus circuit for a device under test and (b) to generate a sampling strobe pulse to sample the response of the device. The strobe pulse has a variable optical path controlled by an optical delay line that is mechanically adjustable, the difference in path lengths determining the sampling time vis a vis the stimulus time.
Another technique is a delta frequency technique described by Brian Kolner and David Bloom ("Electrooptic Sampling in GaAs Integrated Circuits" IEEE J. Quantum Electron., Vol. QE-22, pps. 79-93, 1986). A pair of frequency synthesizers generate a first frequency for synchronizing an optical pulse source, and a second frequency that is an integer multiple of the first frequency plus a small offset, such as one Hertz. The first frequency is used to sample a device under test while the device is stimulated by an electrical signal at the second frequency. The reflected optical energy from the device is detected and the polarization of the return is a measure of the voltage of the device response at the sample time. However, the synthesizers are complex and expensive, especially due to the fractional N synthesizing techniques that provide the small offset for the sampling frequency.
What is desired is an inexpensive means for automatically and precisely controlling the stimulus/response signals for a device under test without the need for expensive frequency synthesizers, mechanical delay stages or trigger generation circuits.